1. Field of the Invention
The present invention relates to active power line conditioners which are utilized to regulate the quality of electrical energy delivered from an electrical energy source to an electrical load. More particularly, the invention relates to an improvement in an active power line conditioner (APLC) which significantly increases peak voltage regulation capability.
2. Description of the Prior Art Electric supply networks are increasingly being exposed to the consequences of nonlinear loads, such as data processing equipment, numerical controlled machines, variable speed motor drives, robotics, medical apparatus and communication equipment. Such loads draw nonlinear pulse-like currents instead of the sinusoidal currents drawn by linear loads (i.e., resistors, inductors and capacitors). These nonlinear currents flow through the source impedance of the electrical energy source, causing distortion of the AC line voltage.
This voltage distortion may produce a number of undesired effects. For example, sensitive loads connected to the network may experience operational difficulties. Additionally, the RMS current supplied by the energy source will generally increase due to the presence of harmonics in the pulse-like currents. These harmonic currents may cause significant resistive (I.sup.2 R) losses in interposing transformers.
Another problem is the occurrence of temporary sags in the AC supply voltage. This may affect, in particular, electrical equipment which utilizes a power supply input stage incorporating a rectifier bridge connected across one or more large filter capacitors. In normal operation, the filter capacitors recharge with each peak of the rectified line voltage. It is only during this peak that the load is actually drawing current from the electrical supply network. When the rectified line voltage is lower than the voltage level on the filter capacitors, the rectifier diodes will prevent current from flowing. If, however, the line voltage does not maintain an adequate peak-no-peak level, these filter capacitors will not be able to maintain their required peak charge levels.
Many of these problems can be mitigated through the use of power electronic devices known as active power line conditioners. Such devices typically comprise one or two switching inverters arranged in a series, parallel, or series-parallel configuration. The inverters are controlled (generally by pulse width modulation (PWM) techniques) to effect a flow of current between a DC energy storage element and the AC supply lines to which they are connected. Such devices are shown and described in U.S. Pat. Nos. 4,651,265 and 3,825,815, which are incorporated herein my reference.
When a single inverter is used, this current may consist of the harmonic and ripple currents required by the load. In a series-parallel configuration, two inverters are arranged to share a common DC link. In this arrangement, the inverters may cooperate to effect a transfer of real power between the source or load and the DC link. This may help to ensure that power delivered to the various loads will be nearly ideal.
The voltage regulation capability of an active power line conditioner is given in terms of a percentage of the nominal AC line voltage. While the line voltage magnitude stays within this rated percentage range, the output voltage stays essentially constant and sinusoidal at the nominal value. Generally, this range is selected to fall between .+-.10% to .+-.25% of the nominal output voltage.
When the supply voltage sinks below the rated boost range of the active power line conditioner, the output voltage tends to also be dragged down. Thus, the line voltage seen by various loads connected to the network will fall below the nominal value and the quality of power delivered to these loads is no longer ensured. This is particularly true in the case of loads having rectified capacitive inputs, since the filter capacitors may not be able to obtain their peak charge level as discussed above. Additionally, transient voltage sags or surges may exceed the selected rating of the series voltage regulator on a statistical basis.
In order, therefore, to provide the greatest assurance of power quality to loads supported by an active power line conditioner, it is desirable for the device to have as large a regulation range as is practical. The weight and cost, however, associated with the magnitude of voltage regulation capability is proportional to the regulation range. As such, an active power line conditioner with a higher regulation range would be expected to be larger and more expensive than a similar device with a smaller regulation range. This may tend to negatively affect the commercial viability of such equipment. The installation of larger components to support excessive input voltage sags may also have greater losses that would reduce overall efficiency.